Few people know this, but on Earth there have been natural nuclear reactors. That is, places in which naturally, a chain reaction did take place. Today it seems that the rarity of the required isotopes and concentrations make this very hard to happen, but for example, traces of such a natural nuclear fission reaction several hundred millions of years ago, in what is today Gabon, in Africa, were found.
In other words, imagine an uncontrolled, uncontained, unstoppable, outdoor Chernobyl mess, and you will get an idea of what it was like then and there.
The following video corresponds to a dissertation about neutrino detection, but at 50 min. from its beginning (approx), the author comments a little bit on this matter.Related: Nuclear reactions Gabon Chernobyl